Intra-cavity frequency converted (frequency doubled) OPS-lasers can provide several Watts (W) of continuous wave (CW) radiation output at visible (green) wavelengths while operating in a single longitudinal mode. The green, single-mode output wavelength can be converted to a wavelength in the ultraviolet (UV) region of the electromagnetic spectrum by further frequency multiplication in an optically non-linear crystal outside the OPS laser cavity (laser-resonator).
By way of example, an OPS-laser having a fundamental lasing wavelength of about 1064 nanometers (nm) can be frequency-doubled by an intra-cavity optically nonlinear crystal to provide output radiation having a wavelength of about 532 nm. That output radiation can be converted to UV radiation having a wavelength of about 266 nm by frequency-doubling the output radiation in an optically nonlinear crystal located outside the OPS laser-resonator.
The doubling of the doubled OPS-laser output radiation t0 266 nm is preferably realized by locating the extra-cavity optically nonlinear crystal in a passive ring-resonator resonant at the doubled frequency (532 nm wavelength). This increases the “green” radiation intensity in the optically nonlinear crystal thereby increasing the conversion efficiency of the crystal. Such a resonator typically has a length which is actively (automatically) controlled by selectively moving one mirror of the ring-resonator while detecting the resonant condition of the ring-resonator thereby maintain the ring resonator in a resonant condition for the OPS-laser output radiation. The resonance-detection method can be a polarization-based method, such as the Hansch-Couillaud method, or frequency-modulation based method such as the Pound-Drever method.
While the OPS-laser output radiation is single longitudinal mode (single frequency) operation environmental factors such temperature change and vibration can cause the single-mode to “hop” from one resonator mode to another. Whenever such a “mode-hop” occurs, the length control mechanism for the ring resonator must accommodate the new single frequency of the OPS-laser output radiation. Even if no mode-hopping occurs, changes in resonator length will cause slight continuous changes in wavelength.
In the case of mode-hopping due to vibration, the mode-hopping can occur at a rate that is comparable to or greater than the response-time of the length-control arrangement for the ring-resonator. This would lead to a generally lower conversion efficiency and noise in the UV radiation generated in the ring resonator. There is a need for an OPS-resonator that is resistant to vibration-induced mode-hopping.